Specimen for Evaluating Pressure Pulse Cavitation in Rock Formations

ABSTRACT

An apparatus  300  for simulating a pulsed pressure induced cavitation technique (PPCT) from a pressurized working fluid (F) provides laboratory research and development for enhanced geothermal systems (EGS), oil, and gas wells. A pump  304  is configured to deliver a pressurized working fluid (F) to a control valve  306,  which produces a pulsed pressure wave in a test chamber  308.  The pulsed pressure wave parameters are defined by the pump  304  pressure and control valve  306  cycle rate. When a working fluid (F) and a rock specimen  312  are included in the apparatus, the pulsed pressure wave causes cavitation to occur at the surface of the specimen  312,  thus initiating an extensive network of fracturing surfaces and micro fissures, which are examined by researchers.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is related to U.S. application Ser. No. ______,filed on ______ and entitled, “A CAVITATION-BASED HYDRO-FRACTURINGTECHNIQUE FOR GEOTHERMAL RESERVOIR STIMULATION”, and U.S. patentapplication Ser. No. 12/945,252 filed on 12 Nov. 2010 and entitled,“REPETITIVE PRESSURE-PULSE APPARATUS AND METHOD FOR CAVITATION DAMAGERESEARCH” the entire contents of which are included herein by referenceas if included at length.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT

This invention was made with government support under Contract No.DE-AC05-00OR22725 awarded by the U.S. Department of Energy. Thegovernment has certain rights in the invention.

THE NAMES OF THE PARTIES TO A JOINT RESEARCH AGREEMENT

None.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present disclosure relates to enhanced geothermal system (EGS)production and particularly to apparatuses and methods for simulating acavitation-based hydro-fracturing technique.

2. Description of the Related Art

Geothermal energy is an important part of the nation's renewable energyinitiative. FIG. 1 illustrates a simplified schematic of a geothermalplant that generates electricity for the electrical grid. A workingfluid (F) such as water is transferred with a pump 100 down into the hotrock formations through an injection well 102, where it absorbs heatenergy from the fractured rock formation. The heated working fluid (F)is then pumped to an energy conversion plant 104 through a productionwell 106. Depending on the fluid's (F) temperature, it may directly beused to power a turbine or may be used to heat a secondary workingfluid, which, in turn, is used to power a turbine. The turbine iscoupled to a generator through a common shaft (not shown), to generateelectricity for the electrical grid 108. The cooled working fluid (F) isthen injected with the pump 100 back into the hot rock geothermalreservoir through the injection well 102 to sustain the process.Geothermal energy generation is considered a green technology, becauselittle or no greenhouse gases are emitted into the atmosphere and theenergy source is renewable.

An Enhanced Geothermal System (EGS) is a man-made reservoir, createdwhere there is sufficient underground hot rock but insufficient orlittle natural permeability or working fluid (F) saturation in the rock.EGS expands the geothermal energy domain into much deeper rock depositsby exploiting natural and artificial fracture systems/networks withinthe rock mass. Maintaining and/or creating such facture networks incomplicated geological environments are critical to the successfuldevelopment and long-term sustainability of the EGS. The EGS targets ahuge energy source that amounts to 500 GWe in the western U.S. and16,000 GWe in the entire U.S. Several demonstration projects areundergoing in the U.S. to validate different reservoir stimulationtechniques. The ultimate reservoir will have a flow rate of 60 kg/s, alifetime of 30 years along the drilling systems down to 10,000 metersdeep at 374 Degrees Celsius.

EGS reservoir stimulation technologies currently are adapted from theoil and natural gas industry including various hydrofracking methodswith or without chemical additives. A potential drawback of usinghydrofracking techniques is the lack of effective control in thecreation of large fractures, which could result in by-pass of targetedfracture network or even fault movement in the rock formation. The lossof hydraulic medium can reduce heat exchange efficiency and increase thecost of the development of EGS. The use of chemicals along with theunpredictable fault movement may also adversely impact the environment.

Cavitation is the process of the formation of vapors, voids, or bubblesdue to pressure changes in a liquid flow as schematically illustrated inFIG. 2. The pressure wave propagation 200, and eventual collapse of thebubbles 202 can cause local pressure changes in the working fluid (F),which can be transmitted to a target rock surface 204 either in the formof a shock wave 206, or by micro-jets 208, depending on the bubble tosurface distance. Pressure greater than 100,000 psi has been measured ina shock wave 206 resonating from cavitating bubbles 202. It is generallyunderstood that the cycle of formation and collapse of the bubbles thatoccurs, often at a high frequency, can generate dynamic stress on thesurfaces of objects. Ultimately, the dynamic stress can contribute tothe fatigue of the target surface, including micro-cracks that form andcoalesce on the surface 204, eventually leading to material removalknown as cavitation damage.

The operations of geothermal, oil and natural gas wells are expensiveendeavors. Well site development and production activities involve vastcapital investments in land and equipment as well as the support ofhighly specialized personnel. Due to these large investments,opportunities for in-situ research and development efforts in thegeothermal, oil and natural gas industries may not be cost prohibitive.

What are needed are apparatuses and methods for simulating apulse-pressure cavitation technique (PPCT) in a laboratory environment.

BRIEF SUMMARY OF THE INVENTION

Disclosed are several examples of apparatuses and methods for simulatinga pulse-pressure cavitation technique (PPCT) in a laboratoryenvironment.

Described in detail below is an apparatus for generating a pulsedpressure induced cavitation technique (PPCT) from a pressurized workingfluid to simulate the hydrofracturing of a specimen when a working fluidand specimen are installed. In the apparatus, a pump is fluidly coupledto, and disposed downstream of, a reservoir and fluidly coupled to, anddisposed downstream of, a control valve having an open position and aclosed position, the pump capable of raising the pressure of a workingfluid at the control valve. Also included is a test chamber for holdinga specimen when a specimen is installed in the apparatus. The testchamber is fluidly coupled to, and disposed downstream of, the controlvalve and receives a working fluid from the control valve when thecontrol valve is in the open position. Also included is a pressureregulator that is fluidly coupled to, and disposed downstream of, thetest chamber and fluidly coupled to, and disposed upstream of, thereservoir. When the control valve is in the open position, it causes aworking fluid to flow into the test chamber as a pressure pulse, causingcavitation to occur in a working fluid adjacent to a specimen when aspecimen and a working fluid are installed in the apparatus. Otherfeatures and examples will be described in greater detail.

Also described in detail below is an article or specimen for receiving apulsed pressure induced cavitation technique (PPCT) from a pressurizedworking fluid as generated by a test apparatus. The specimen includes ashell body defined by a circular top surface, a circular bottom surfaceand a convex side surface joining the top and bottom. Also included inthe shell body is an aperture defined by an opening in the top surfaceand an opening in the bottom surface. Also included is a core bodydefined by a top surface, a bottom surface and a convex side surfacejoining the top surface and bottom surface. The core body is disposedinside of the aperture in the shell body and the shell body and corebody are made of rock materials. Other features and examples will bedescribed in greater detail.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The apparatus and method may be better understood with reference to thefollowing non-limiting and non-exhaustive drawings and description. Thecomponents in the figures are not necessarily to scale, emphasis insteadbeing placed upon illustrating principles. In the figures, likereferenced numerals may refer to like parts throughout the differentfigures unless otherwise specified.

FIG. 1 is a simplified sectional schematic of a geothermal energyconversion plant.

FIG. 2 is a simplified rendition of cavitation mechanics at a fluid andsurface interface.

FIG. 3 is a plan view of an exemplary apparatus for simulating acavitation-based hydro-fracturing of a specimen.

FIG. 4 is an illustration of another exemplary apparatus for simulatinga cavitation-based hydro-fracturing of a specimen.

FIG. 5 is an external view of an exemplary control valve for use withthe apparatuses of FIGS. 3 and 4.

FIG. 6 is a sectional view of the control valve of FIG. 5.

FIG. 7 is an illustration of the internal elements of the control valveof FIG. 5.

FIG. 8 is a sectional view of a test chamber as used with the controlvalve of FIG. 5.

FIG. 9 is an illustration of an exemplary test chamber withinstrumentation and dual control valves installed.

FIG. 10 is an illustration of an exemplary heating device for use withthe test chambers.

FIG. 11 is an illustration of an exemplary specimen.

FIG. 12 is an illustration of another exemplary specimen.

FIG. 13 is an illustration of an exemplary specimen shell.

FIG. 14 is an illustration of an exemplary specimen core for use withthe shells of FIGS. 13 and 15.

FIG. 15 is an illustration of another exemplary specimen shell.

DETAILED DESCRIPTION OF THE INVENTION

With reference now to FIG. 3, an exemplary apparatus 300 for generatinga pulsed pressure induced cavitation technique to simulate thehydrofracturing of a specimen will now be described in greater detail. Aworking fluid (F), such as water, hydraulic fluid, other fluid, orcombination of fluids, is stored in a reservoir 302. The reservoir 302may be an open or closed vessel and may also include means forfiltering, adding, removing, and/or monitoring the level of workingfluid (F). A pump 304 draws the working fluid (F) from the upstreamreservoir 302 and distributes it to one or more downstream controlvalves 306 at pressures less than or equal to approximately 300 psi(2068.4 kPa), greater than or equal to approximately 300 psi (2068.4kPa), or greater than or equal to approximately 300 psi (2068.4 kPa) andless than or equal to approximately 2,000 psi (13789.5 kPa). The pump304 may operate by compressed air or by an electric motor for example.An air operated liquid piston pump 304 from Haskel International, Inc.Burbank, Calif., 91502 is suitable for this particular application.

The control valve 306 receives the pressurized working fluid (F) fromthe upstream pump 304 and delivers it to a downstream test chamber 308.High speed compressed air or electric solenoid valves may be used forthe control valve 306. A programmable controller (not shown) is used tocontrol the timing frequency of the opening and closing of the controlvalve 306 to suit each particular simulation. A laptop or desktopcomputer using LabVIEW software by National Instruments, or a similarcontroller and software product, may be used. In some examples, thecontroller may signal the control valve 306 to open and close at apredetermined open and close frequency and/or duration schedule.Frequencies less than or equal to approximately 300 cycles per minute,greater than or equal to approximately 300 cycles per minute, or greaterthan or equal to approximately 300 and less than or equal toapproximately 60,000 cycles per minute may be used. In other examples,the controller may signal the control valve 306 to remain in the openposition for a period of time. Although a single control valve 306 isillustrated in FIGS. 3 and 4, two or more control valves 306 may also beused as shown later in FIG. 9.

The exemplary test chamber 308 receives the pressurized working fluid(F) from the upstream control valve 306. In this embodiment, the testchamber 308 is a cylindrical shaped tube defining an internal cavitationchamber 310 for accepting a test specimen 312. This example of a testchamber 308 has an upstream end cap 314 that is fluidly coupled to theupstream control valve 306, a medial body 316 and a downstream end cap318. The term fluidly coupled refers to a system where the fluid is ableto flow between one component and another. At least one of the end caps314, 318 are removable from the body 316 to allow for loading andunloading of a specimen 312 into the cavitation chamber 310.Corresponding threads 320 on the end caps 314, 318, and body 316cooperate to provide a fluid seal when assembled together (see FIG. 8).The end caps 314, 318 and body 316 are machined from high-strength,corrosion-resistant material such as stainless steel for example. SAE304 or SAE 316 stainless steel perform well in this application. Othersuitable materials may also be used.

A fluid pressure regulator 322 may be fluidly coupled between the testchamber 308 and the reservoir 302. The pressure regulator 322 may be adiaphragm type, for example, and may contain an integral pressure gauge324 for ensuring accurate adjustments to the fluid pressure in thesystem. As is typical in such regulators, a clockwise turn of theadjustment knob increases system pressure and a counterclockwise turnreduces system pressure. One or more pressure gauges 324 may beinstalled at different locations in the system to ensure proper workingfluid (F) pressure.

Referring now to FIG. 4, another exemplary apparatus 300 for generatinga pulsed pressure induced cavitation technique to simulate thehydrofracturing of a specimen will now be described in greater detail.In this example, a pressure accumulator 326 may be fluidly coupledbetween the pump 304 and the control valve 306. The pressure accumulator326 may be a gas charged type, a bellows type, or other type of pressureaccumulator known in the art. The pump 304 delivers the working fluid(F) to the accumulator 326, raising its pressure, until the controlvalve 306 is opened. All other components and features of this exemplaryapparatus 300 are as described above.

Conduits 328 are used to fluidly couple each of the components togetherand direct the working fluid (F) between components. High pressurecapacity conduits 328 made of stainless steel may be used. Suitablecouplings such as flared end fittings, or AN style fittings may be usedto join the conduits 328 to the individual components described above.

Referring now to FIGS. 5-8, another exemplary control valve, alsoreferred to as a rotary shutter valve 500, will be described in greaterdetail. In this example, an outer housing 502 includes an upstream end504, an opposite downstream end 506, and a medial portion 508 disposedbetween the two ends. The outer housing 502 is preferably made from twocylindrical-shaped segments that are joined together at acircumferential flange 510 to simplify assembly, cleaning, inspection,modification, and repair of the valve. The flange 510 is held togetherwith a plurality of circumferentially spaced fasteners 512 such asrivets, clamps or threaded fasteners as shown. An O-ring type seal 514engages a corresponding gland machined in one or both of the segments asillustrated in FIG. 6. The outer housing 502 is machined from a highstrength, high temperature and corrosion resistant material such asstainless steel. SAE 304 or SAE 316 stainless steel performs well inthis application.

An inlet aperture 516 is defined by the outer housing 502 at itsupstream end 504. An integral boss 518 provides additional material forconnecting a conduit 328 using fittings as described above. The inletaperture 516 is fluidly coupled to a pressure chamber 520, which is alsodefined by the outer housing 502 at its upstream end 504. The workingfluid (F) flows under pressure from the pump 304, though the conduits328 to the inlet aperture 516, and into the pressure chamber 520. Thedownstream end 506 of the outer housing 502 defines a pulse cavity 522,which discharges the pressurized working fluid (F) from the rotaryshutter valve 500 as a series of pressure pulses 200 into the testchamber 308 (FIG. 8).

The medial portion 508 of the outer housing 502 defines a bulkhead 524,which separates the pressure chamber 520 from the pulse cavity 522. Thebulkhead 524 is preferably made integral with the outer housing 502, butit may also be a separate component that is joined to the outer housing502 by threads or other mechanical means such as welding. The bulkhead524 defines one or more bulkhead apertures 526, which fluidly couple thepressure chamber 520 with the pulse cavity 522. In the example shown,two, equally spaced, circular bulkhead apertures 526 are used. In otherexamples, more or less apertures 526 of circular or other shapes areused. Also, apertures 526 with constant (shown), converging, ordiverging cross sections from their upstream to downstream ends arecontemplated. The upstream surface 528 of the bulkhead 524 is planarshaped and the downstream surface 530 is concave conical shaped in theexample. The concave conical shape of the downstream surface helpsdirect the pressure waves 200. Other shapes (e.g., concave spherical,concave parabolic) are contemplated for the bulkhead downstream surface530 as well.

A rotatable shutter 532 is disposed inside of the pressure chamber 520and adjacent to the upstream surface 528 of the bulkhead 524. Theshutter 532 defines one or more windows 534 that generally conform insize, shape, and radial placement with the bulkhead apertures 526. Inthe example shown, four, equally spaced, circular windows 534 are used.In other examples, more or less windows 534 of circular or other shapesand sizes are used. The shutter 532 is affixed to, or integral with, ashaft 536 that extends through the pressure chamber 520 and exits theouter housing 502 at its upstream end 504.

Thrust bearings 538 support the shaft 536 and fit in pockets machined inthe bulkhead 524 and the upstream end 504 of the outer housing.Shoulders on the shaft 536 engage with the thrust bearings 538 toprevent the shaft 536 from moving axially, thus preventing the shutter532 from contacting the bulkhead 524, seizing, and/or causingdestructive vibrations while rotating. An O-ring type seal 540 engages acorresponding gland machined in the radially outer surface of theshutter 532 and prevents leakage of the working fluid (F) from the gapbetween the shutter 532 and the outer housing 502. A material such aspolyurethane, aluminum, graphite or other strong, high temperaturecapable material may be used for the O-ring seal 540.

Extending outward from the upstream end 504 of the outer housing 502 isa mounting flange 542 for accepting a powering device 544. The poweringdevice 544 is affixed to the mounting flange 542 with one or morefasteners 546 such as rivets, bolts or screws. In the example shown, anelectric motor is used as the powering device 544, but a hydraulicmotor, a pneumatic motor, or other such device would also work in thisapplication. Electricity, air, or hydraulic fluid is supplied to thepowering device 544 by wires or hoses respectively (not shown).

A coupling 548 connects the powering device 544 to the shaft 536. Thecoupling 548 may include threads, set screws, shear pins, keys, collets,and/or other connecting means. In order to protect the powering device544 from damage, the coupling 548 is designed to fail if the shutter 532and/or shaft 536 break, seize, or become otherwise jammed in thepressure chamber 520 for some reason.

During operation of the rotary shutter valve 500, the powering device544 transfers rotation to the shaft 536 through the coupling 548. Thespinning shaft 536 rotates the shutter 532, causing the windows 534 toalternately align with (unblock) and misalign with (block) the one ormore bulkhead apertures 526. The pressurized working fluid (F) in thepressure chamber 520 discontinuously flows through the apertures 526,into the pulse cavity 522, and out of the downstream end 506 as pressurepulses 200. The pressure pulses cause cavitation to occur in the testchamber 308 and, in turn, introduce fractures and micro cracks in a testspecimen 312 when a test specimen is installed. It is noted that thepulses 200 are controlled by the number and size of the bulkheadapertures 526, the number and size of shutter windows 534, therotational speed of the shutter 532, and the pressure of the workingfluid (F). The shutter 532 can rotate at speeds less than or equal toapproximately 300 revolutions per minute, greater than or equal toapproximately 300 revolutions per minute, or greater than or equal toapproximately 300 revolutions per minute and less than or equal toapproximately 60,000 revolutions per minute (RPM).

In this example, the test chamber 308 receives the pressurized workingfluid (F) directly from the pulse cavity 522 of the rotary shutter valve500. The test chamber 308 is a cylindrical shaped tube defining aninternal cavitation chamber 310 for accepting a test specimen 312. Thisexample has a medial body 316 and a downstream end cap 318. The testchamber 308 is attached to the distal end 506 with threads or otherfeatures to allow for loading and unloading of the specimen 312. Otherfeatures of the present test chamber 308 are as described in the earlierexamples.

FIG. 9 shows another example of a test chamber 308 including instruments330 for monitoring the conditions inside the cavitation chamber 310 suchas the temperature, pressure and flow rate of the working fluid (F). Itis also noted that, in this particular embodiment, two control valves306 are fluidly coupled to the test chamber 308 at the upstream end cap314 with each valve 306 functioning as described above with respect toFIGS. 3 and 4. In this example, the working fluid (F) pressure pulsesentering the test chamber 308 are directly controlled by the frequencyand/or duration schedule(s) of the control valve(s), which may beprogrammed to open and close according to the same schedule or accordingto different schedules.

Referring now to the example of FIG. 10, the test chamber 308 may besurrounded, at least partially, by a heating element 332 to simulate theelevated temperature found in a EGS reservoir, or an oil or gas well. Inthe example shown, a resistance heater 332 completely surrounds the testchamber 308, but in other examples only a portion of the chamber issurrounded by a heater. In some examples, the heater is able to raisethe temperature of the test chamber 308 and specimen 312 to atemperature less than or equal to approximately 50 degrees Celsius (122Fahrenheit), greater than or equal to approximately 50 degrees Celsius(122 Fahrenheit), or greater than or equal to approximately 50 degreesCelsius (122 Fahrenheit) and less than or equal to approximately 250degrees Celsius (482 Fahrenheit).

Referring lastly to FIGS. 11 and 12, exemplary specimens 312 forevaluating pressure pulse cavitation in EGS, oil or gas well rockformations are shown. The specimens 312 are generally cylindrical inshape and defined by a circular top surface 334, a circular bottomsurface 336 and a convex side surface 338 extending between the top andbottom surfaces 334, 336. The specimens 312 are comprised of rock orstone material from larger rock specimens of the type found in EGSreservoirs, oil or gas wells. They are machined or core drilled to shapeand sized to fit within the test chamber 308.

In the example of FIG. 11, a blind hole 340 mimics a stimulation well.During testing, the hole 340 is filled with working fluid (F) and issubject to cavitation by controlling the opening frequency and durationof the control valve(s). In the example of FIG. 12, a series ofartificial flaws 342 are included in the side surface 338. Here,artificial flaws 342 such as cracks or fissures are introduced into theside surface 342 with a band saw, a water jet or other cutting device tosimulate an existing crack structure and/or to assist the initiation ofcrack stimulation.

In the examples of FIGS. 13-15, a shell body 344 is defined by agenerally circular top surface 334, a circular bottom surface 336 and aconvex side surface 338 joining the top and bottom surfaces 334, 336. Anaperture 346, having an interior surface 348, is disposed through theshell body 344 and is defined by a circular opening in the top andbottom surfaces 334, 336. A separate, core body 350 is defined by acircular top surface 334, a circular bottom surface 336 and a convexside surface 338 joining the top and bottom surfaces 334, 336. Theaperture 346 of the shell 344 is sized to accept the core 350 therein.As in the previous examples, the specimens 312 are comprised of rock orstone material of the type found in EGS reservoirs, oil or gas wells.They are machined or core drilled to shape and sized to fit within thetest chamber 308.

In the present examples, artificial flaws 342 (surface flaws or throughthickness flaws) may be introduced into one or both of the shell 344 andcore 350. During testing, cavitating working fluid (F) is forced to flowalong the interface between the shell 344 and the core 350. Furthermore,by incorporating a 45 degree pitch spiral notch to the side surface 338of the core 350 and/or a spiral through thickness notch to the sidesurface 338 of the shell 344, these specimens 312 can also be used toevaluate the fracture toughness degradation during the EGS reservoiroperation.

Further information about a spiral-notch torsion test system (SNTT) maybe found in U.S. Pat. No. 6,588,283, “Fracture Toughness DeterminationUsing Spiral-Grooved Cylindrical Specimen and Pure Torsional Loading”,to Jy-An Wang and Kenneth C. Liu, the disclosure of which is herebyincorporated by reference. Additional information may also be found in“A New Test Method for Determining the Fracture Toughness of ConcreteMaterials” by J. A. Wang, K. C. Liu, D. N. in Cement and ConcreteResearch, Volume 40, Issue 3, March 2010, Pages 497-499, K. Scrivenereditor. Such benchmark data can further provide the guideline on thestimulation pressure pulse design parameters and their effectiveness forgenerating crack growth.

After simulation testing, the specimens 312 are examined in order toevaluate the fracture network caused by the pressure pulse cavitationtechnique by the apparatus 300. It was found that two main mechanismsoccur in cavitation erosion damage: high pressure shock waves created bythe collapsing vapor bubbles, which can result in material fatigue andplastic deformation; and micro-jet impingement resulting in asymmetricalcollapse of the vapor bubble near the specimen 312 surface. It was alsofound that when the bubbles collapse due to external pressure, theworking fluid (F) is accelerated toward the center of the bubble.Bubbles formed near solid surfaces have the largest potential to causemicro cracking of the specimen 312 surface.

While this disclosure describes and enables several examples of asimulator apparatuses and methods for researching geothermal reservoirstimulation, other examples and applications are contemplated.Accordingly, the invention is intended to embrace those alternatives,modifications, equivalents, and variations as fall within the broadscope of the appended claims. The technology disclosed and claimedherein may be available for licensing in specific fields of use by theassignee of record.

1-17. (canceled)
 18. An article for receiving a pulsed pressure inducedcavitation effect from a pressurized working fluid as generated by atest apparatus, the article comprising: a shell body defined by acircular top surface, a circular bottom surface and a convex sidesurface joining the top and bottom; an aperture disposed through theshell body and defined by an opening in the top surface and an openingin the bottom surface; a core body defined by a top surface, a bottomsurface and a convex side surface joining the top surface and bottomsurface; and wherein the core body is disposed inside of the aperture inthe shell body and the shell body and core body are made of rockmaterials.
 19. The article of claim 18 and further comprising: a45-degree pitch spiral notch in the side surface of the core body. 20.The article of claim 19 and further comprising: a 45-degree pitch spiralnotch through the thickness of the shell body.
 21. The article of claim18 wherein the aperture in the shell body is sized to accept the corebody and to allow cavitating working fluid to flow through the interfacebetween the shell body and the core body.
 22. The article of claim 18wherein at least one of the shell body and the core body includes anartificial flaw.
 23. The article of claim 22 wherein the core bodyincludes a blind hole artificial flaw in the top surface.
 24. Thearticle of claim 22 wherein the core body includes a crack or fissureartificial flaw in the side surface.
 25. An article for receiving apulsed pressure induced cavitation effect from a pressurized workingfluid as generated by a test apparatus, the article comprising: aspecimen body made of rock material and defined by a top surface, abottom surface and a convex side surface joining the top surface andbottom surface; and wherein a surface of the body includes an artificialflaw.
 26. The article of claim 25 wherein the surface is a top surfaceand the artificial flaw is a blind hole.
 27. The article of claim 25wherein the surface is a convex side surface and the artificial flaw isa crack or fissure.